The purpose of the present study is development of a powerful numerical method for calculating second-order diffraction loads on plural vertical cylinder with arbitrary cross sections. According to the present method, second-order wave force can be obtained from a linear radiation potential without solving second-order boundary value problem. The boundary value problem for the radiation potential is solved with the hybrid boundary element method. The computations for circular and rectangular cylinders were carried out and compared with the experiments. In addition, second-order wave forces on twin circular cylinder are calculated with the present method.